Solar Radiation Effects on Pyrgeometer Performance

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Abstract
In 1973 an Eppley Laboratory pyrgeometer was installed on a field site in St. Paul, Minn., to measure atmospheric radiation (Ld). Based on comparisons between measured and calculated Ld for clear nights and low overcast conditions during days and nights the pyrgeometer provided satisfactory measurements. However, on clear days measured Ld was found to range from 11–20% higher than calculated. When the pyrgeometer dome was shaded from direct solar rays there were reductions in output of about 20–120 W m−2. Calculations showed that transmission of solar radiation by the pyrgeometer dome accounted for a maximum of about 7 W m−2 of this amount. Thus, it was inferred that the high values of measured Ld were primarily caused by solar heating of the dome and its subsequent radiation to the thermopile. This was strengthened by the fact that on clear windy days there were short-term fluctuations in output of 15–35 W m−2. It was determined that these fluctuations were caused by wind gusts cooling the dome. A ventilation system that supplied 0.40 m3 min−1 of air flow across the dome reduced the short-term fluctuations from 15–35 W m−2 to 7 W m−2. When the ventilated pyrgeometer was shaded reductions in output amounting to 15–40 W m−2 were obtained. A second ventilation system that supplied 0.85 m3 min−1 of air did not result in any additional reduction in the dome heating effect. Results from this study indicate that the total daily Ld measured by the pyrgeometer at St. Paul would be about 5 and 9% too high for typical clear winter and summer days, respectively. With ventilation of the pyrgeometer the total daily Ld values for average clear winter and summer days would be only about 2.5 and 3.6% too high, respectively.

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